This invention relates to the fabrication of very-high-resolution devices and, more particularly, to mask structures for use in an x-ray lithographic system.
One of the keys to the realization of a commercially feasible x-ray lithographic system is the construction of a suitable master mask which comprises a substrate having a pattern of x-ray-absorptive elements formed thereon. Various materials have been suggested for making the substrate of such a mask. One such material is beryllium, which is characterized by low x-ray absorption but which is expensive, optically opaque (which makes alignment and registration difficult) and toxic. In addition, silicon structures having thin x-ray-transparent windows have been fabricated, but such structures are relatively fragile and optically transparent only to a partial extent.
The use of a thick membrane of Kapton polyimide film (about 25 micrometers thick) as a relatively rugged x-ray mask substrate is described in U.S. Pat. No. 4,037,111, issued to G. A. Coquin, J. R. Maldonado and D. Maydan. (Kapton is a registered trademark of E. I. duPont de Nemours and Co.). In some applications, however, it has been found that thermally induced dimensional changes in such a Kapton film cause unacceptable distortions to occur in the pattern formed on the film.
In a commonly assigned copending application of A. C. Adams, C. D. Capio, H. J. Levinstein, A. K. Sinha and D. N. K. Wang, Ser. No. 941,776, filed Sept. 13, 1978, other mask structures for x-ray lithography are described. The masks described therein comprise a substrate made, for example, of boron nitride bonded to a support frame. Such a mask structure exhibits an advantageous substantially distortion-free characteristic. But in practice a boron nitride substrate is exceedingly fragile, difficult to handle and prone to breakage. Moreover, in some applications of importance, the surface quality of such a substrate is as a practical matter insufficiently high to serve as a basis for forming very-low-defect-density patterns thereon.
Accordingly, continuing efforts have been directed at trying to devise a mask structure which would exhibit all or most of the aforementioned desirable characteristics without any of the disadvantages of masks as heretofore constructed. It was recognized that such a mask, if available, would remove one of the major practical obstacles to realizing a rugged production-type x-ray lithographic system suitable for fabricating very-high-resolution devices.